In internal combustion engines, it is under certain conditions possible for an uncontrolled combustion or a self-ignition of the residual mixture to occur following the actual ignition and intended firing of the mixture, which is called engine knock. Knocking combustions cause great mechanical and thermal stress in the engine, which can result in damage.
To prevent engine knock or to return to a knock-free operation of the internal combustion engine following the detection of engine knock and in order to be able to operate the engine as closely as possible to the knock limit, a so-called knock control is used, in which usually the ignition timing is retarded. Aside from further measures such as reducing the load or enriching the fuel mixture, conventionally, cooling and explosion-reducing substances are injected such as water for example. By way of the water injection, in which preferably distilled water or a water/alcohol mixture is injected into the combustion chamber or the intake manifold of the internal combustion engine, it is possible to reduce the knock tendency or to shift the knock limit so that it is possible to operate the engine at a more optimal operating point.
The water injection causes inter alia a cooling of the mixture and an interior cooling of the engine and has an influence on the course of the combustion. Because of the colder combustion air and thus the latter's greater density, the water injection is able to achieve an increase in performance. The latter is in most cases the reason why water injection is used in a vehicle.